The primary experiments will involve measurements of infarct size in dog hearts twenty-four hours after acute occlusion of the left anterior descending coronary artery using experimental procedures recently developed in our laboratory to quantitate the size of the infarct relative to the amount of tissue jeopardized by coronary occlusion. Infarct size will be calculated from myocardial creatine phosphokinase and by planimetry of photographs of the sectioned heart. The size of infarction that would occur in untreated animals will be predicted from measurements of coronary collateral blood flow using radioactive labelled 15u diameter microspheres, and the difference between predicted and actual infarct sizes will quantitate the amount of salvaged myocardium. The second objective of the proposed project is to identify the mechanism of action of interventions altering infarct size by separating the role of changes in collateral blood flow from other factors influencing myocardial necrosis.